Figure 4. Color-color scatter diagrams
showing that the XO-4 star field has similar color relationships
to those in the set of Landolt stars, thus validating the quality of
the all-sky observations reported here.

These are "internal consistency checks." It passes!

I conclude that interstellar reddening biases the Tycho V-mag
value, and might also influence the conversion from J and K to V.
The two all-sky measurements provide a consistent result that is free
of the interestellar reddening biases. which argues for V-mag = 9.80
± 0.03.